Process for the facile electrosynthesis of graphene from CO2

ABSTRACT

The present invention relates to the production of graphene from CO2 through electrolysis and exfoliation processes. One embodiment is a method for producing graphene comprising (i) performing electrolysis between an electrolysis anode and an electrolysis cathode in a molten carbonate electrolyte to generate carbon nanomaterial on the cathode, and (ii) electrochemically exfoliating the carbon nanomaterial from a second anode to produce graphene. The exfoliating step produces graphene in high yield than thicker, conventional graphite exfoliation reactions. CO2 can be the sole reactant used to produce the valuable product as graphene. This can incentivize utilization of CO2, and unlike alternative products made from CO2 such as carbon monoxide or other fuels such as methane, use of the graphene product does not release this greenhouse gas back into the atmosphere.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of Ser. No. 16/886,409, filed May 28,2020, which claims the benefit of U.S. Provisional Nos. 62/938,135,filed on Nov. 20, 2019, 62/890,719, filed on Aug. 23, 2019, and62/853,473, filed on May 28, 2019, the entire contents of each of whichare incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to the production of graphene from CO₂through electrolysis and exfoliation processes. The exfoliating stepproduces graphene in high yield than thicker, conventional graphiteexfoliation reactions. CO₂ can be the sole reactant used to produce thevaluable product as graphene. This can incentivize utilization of CO₂,and unlike alternative products made from CO₂ such as carbon monoxide orother fuels such as methane, use of the graphene product does notrelease this greenhouse gas back into the atmosphere.

BACKGROUND OF THE INVENTION

Graphene has unique properties that are useful for a variety ofapplications. However, the synthetic costs and the challenge to isolatethe graphene product in its native two dimensional structure lead to thehigh current cost of graphene, valued at approximately $1 million perton. See Price and Market of Materials, Carbon XPrize Standards DataSummary Set, Draft V1.2, (Sep. 12, 2017)).

Graphene has a high surface area, high thermal and electricalconductivity, strength, surface tailorability, and high charge carrierconductivity that makes it uniquely suitable for energy storage andelectronics. See, e.g., Coroş et al., Front. Mat. Sci., 2019, 13, 23;Agudosi et al., Crit. Rev. Mat. Sci., 2019, 1040-8436, 1; Bai et al.,Electrochem. Energy Rev., 2019, doi.org/10.1007/s41918-019-00042-6; andZhang et al., Adv. Sci., 2017, 1700087, 4.

The ability of graphene to carry plasmons allows it to strongly interactwith light in a non-linear fashion and act both as a transducer andtransmitter in optoelectronics. Graphene's 2D honey-comb lattice sp²crystal structure possess extremely high intrinsic charge mobility(250,000 cm²/Vs), a high specific surface area (2630 m²/g), high thermalconductivity (5000 W/mK), high Young's modulus (1.0 TPa), and highoptical transmittance (97.7%).

Methods to produce graphene include thermal annealing (see, e.g., Li etal., J. Nanomat., 2011, 2011, 319624), unzipping nanotubes (see, e.g.,Tanaka et al., Sci. Rep., 2015, 5, 12341), solvothermal and thermaldecomposition (see, e.g., Singh et al., Int. J. Nanosci., 10, 39; Bergeret al., J. Phys. Chem. B, 2004, 108, 19912), ball-milling and chemicalexfoliation (see, e.g., Del Tio-Castillo et al., Nano Res., 2014, 7,963; Liu et al., Chem. Eng. J., 2019, 355, 181), and chemical vapordeposition (CVD) (see, e.g. Shukla et al., Appl. Phys. Rev., 2019, 6,021331; Azam et al., ECS J Solid State Science Technology, 2017 6(6)M3035; Lee et al., RSC Adv, 2017, 7, 15644; and Zhang et al., Adv. Sci.,2017, 1700087, 4).

Chemical vapor deposition (CVD) is a popular method to produce graphenefrom a variety of organometallics or other carbon sources usingtransition metal catalysts. However, conventional CVD can have a massivecarbon footprint of over 600 tonnes of CO₂ per tonne of nano-carbonproduced (see, e.g. Khanna et al., J. Ind. Ecology, 2008, 12, 394).

In a 2003 paper investigating processes detrimental to Li-ion batteryanodes, it was noted that electrochemical alkali ion intercalation couldlead to peeling off of layers from a graphite anode (see, e.g., Buqa etal., US DOE Tech Rep,

ETDE-CH-0301, 2003, 63; also see Spahr et al., J. Electrochem. Soc.,151, 2004, 181).

In 2007, the observation of one-atom thick graphene layers byelectrochemical exfoliation was observed (see, e.g., Penicaud et al.,Compos. Sci. Technol., 67, 2007, 795; Mansour et al., Carbon, 45, 2007,1651; and Valles et al., J. Am. Chem. Soc., 130, 2008, 15802).

Electrochemical exfoliated graphene prepared from graphite is ofincreasing interest today, and is often mechanistically interpreted asan anodic process in which intercalated ions between the graphite layersare oxidized, forming gases which break the weak interlayer Van derWaals bonds, and release thin single or multi-layered graphene sheetsinto the electrolyte (see, e.g., Hashimoto et al., Electrochem. Comm.,104, 2019, 106475; Xia et al., Nanoscale, 11, 2019, 5265; Bakunin etal., Inorg. Mat.: Appl. Res., 10, 2019, 249; and Khahpour et al., Appl.Energy Mat., 2, 2019, 4813). In 2017, it was observed that compressionof graphite flakes prior to exfoliation, such as using graphite powderconfined by wax coating could increase the yield of graphene (see, e.g.,Wang et al., Appl. Mat. Interfaces, 9, 2017, 34456).

A low carbon footprint carbon nanomaterial may be produced from a moltencarbonate by electrolysis, at low cost and using CO₂ as a reactant, forexample as a C2CNT (CO₂ to Carbon Nanotube) synthesis. However,technical challenges have prevented scale-up of the process and thenanomaterial remains scarce. While examples of carbon nanotubes (CNTs)prepared by C2CNT synthesis have been termed “straight,” each example ofsynthesized, grouped, CNTs shown was visibly entangled, and twisted orhooked, although less twisted than CNTs denoted “tangled”. Entangled andtwisted CNTs tend to agglomerate and are it is difficult therefore todisperse them homogeneously in a composite. In the C2CNT synthesizedexamples “straight” referred specifically to CNTs containing less sp³bonding amongst carbons defects, and “tangled” CNTs contain more sp³defects. Example processes for producing carbon nanomaterials frommolten carbonates by electrolysis are disclosed in, for example, Lichtet al., J. CO ₂ Utilization, 2017, vol. 18, 335-344; Nano Lett., 2015,vol. 15, 6142-6148; Materials Today Energy, 2017, 230-236; Data inBrief, 2017, vol. 14, 592-606; Scientific Reports, Nature, 2016, vol. 6,1-10; ACS Cent. Sci., 2015, vol. 2, 162-168; RSC Adv., 2016, vol. 6,27191-27196; Carbon, 2016, vol. 106, 208-217; Energy Conyers. Manag.,2016, vol. 122, 400-410; J. CO ₂ Utilization, 2017, vol. 18, 378-389; J.CO ₂ Utilization, 2017, vol. 18, 335-344; J. Phys. Chem. Lett., 2010,vol. 1, 2363-2368; J. Phys. Chem. C, 2009, vol. 113, 16283-16292; J. CO₂ Utilization, 2019, vol. 34, 303-312; Adv. Sustainable Syst., 2019,vol. 3, 1900056; and Mater. Today Sustainability, 2019, vol. 6, 100023;U.S. Pat. Nos. 9,758,881 and 9,683,297, U.S. Publication No. 2019/36040,and International Publication Nos. WO 16/138469, WO 18/093942, and WO18/156642.

There remains, however, a need for a convenient and facile low cost, lowcarbon footprint synthesis of graphene.

SUMMARY OF THE INVENTION

The present invention describes a novel facile electrosynthesis ofgraphene at low cost from CO₂. The process involves (i) performingelectrolysis between an electrolysis anode and an electrolysis cathodein a molten carbonate electrolyte to generate carbon nanomaterial on thecathode; and (ii) electrochemically exfoliating the carbon nanomaterialfrom a second anode to produce graphene.

The electro-synthesized carbon platelets are nano-thin, promoting highergraphene yields than observed using thicker, conventional graphiteexfoliation processes. CO₂ can be the sole reactant used to produce thegraphene product. Utilization of CO₂ as the sole reactant producesgraphene as a low carbon footprint product. This incentivizesutilization and consumption of CO₂ and, unlike alternative products madefrom CO₂ such as carbon monoxide or other fuels such as methane, use ofthe graphene product does not require combustion and does not releasethis greenhouse gas back into the atmosphere. The cost of theelectrochemical processes described herein is low and carbon dioxide isconsumed in the formation of the graphene. Prior to the work describedherein, it was considered that graphene could only be mass produced witha high carbon footprint and at high cost. CO₂ electrolysis in moltencarbonate production of carbon platelets readily scales upward linearlywith the area of the electrolysis electrodes, facilitating larger scalesynthesis of graphene.

The graphene produced by the processes described herein typicallyexhibits a relatively small lateral dimension (on the order of about 2to 8 μm). This lateral size is beneficial, for example, for the use ofgraphene as a lubricant, in battery anodes, and in graphene admixtureapplications. Larger lateral dimensions, however, may be expected withfurther variations in the electrochemical growth parameters, including,for example, electrolysis duration, current density, temperature,electrode and electrolyte composition, and would expand the utility ofthe molten carbonate electrolysis processes described herein.

Electrosynthesized carbon platelets and other non-CNT graphene layeredmorphologies (such as carbon nano-onions) comprising nano graphenelayers in unique arrangements may be synthesized by the processesdescribed herein. The inventor has discovered the molten carbonateelectrosynthesis of two classes of carbon nano-products. A first classis formed when a transition metal nucleating agent is included in theelectrolysis and produces carbon nanotubes and carbon nanofibers. In thepresent invention, a second class is formed when transition metalnucleating agents are suppressed or excluded from the electrolysis,yielding unique nano structures including, for example, nano-platelets,nano-onions and nano-scaffolds. Each of the nano-structures describedherein contains layered graphene and may be exfoliated to form grapheneplates.

Without being bound to any particular theory, the present inventortheorizes that carbon nanotubes are thermodynamically more stable andgrow more readily than other graphene layered nanomaterial products. Oneramification of this stability is that CNTs display the highest materialstrength of any material measured to date. See e.g., Yu, et al.,Science, 287 (2000) 637-640 and Chang et al., ACS Nano 4 (2010)5095-5100. Hence, CNTs provide a low energy route to a specific carbonnanomaterial product.

Nanotube growth in molten carbonate is electrocatalytically facilitatedby transition metal nucleation. When the nucleation is disrupted by, forexample, suppression, exclusion and/or inhibition, alternative carbonnano morphologies are observed to occur. In order to support thedominant growth of unique graphene layered carbon nano-nano-scaffolds,an experimental set of conditions have been identified that discouragethe transition metal nucleation route. For example, several electrolysisconditions are described herein that reliably and consistently inhibitCNT nucleation and promote growth of other graphene layered based carbonnano-materials, even in the presence of the transition metal nucleationagents, such as Ni, Cr and Fe.

The first is the direct cathodic deposition exclusion of transitionmetals that can be in the electrolysis system (e.g., the deposition oftransition metals onto the cathode is inhibited, suppressed orprevented). For example, this can be achieved by selecting electrolyticconditions which suppress the solubility of transition metal nucleatingagents in the electrolyte. The lowered solubility minimizes theirconcentration in the electrolyte or near the cathode surface to inhibittheir diffusion and inhibit the development of nucleation seeds requiredfor CNT growth. Examples of these physical chemical conditions include,for example, (i) the use of nucleating metals, such as iron, in binarycarbonates (i.e., a mixture of carbonates such as lithium carbonate incombination with potassium and/or sodium carbonate, instead of purelithium carbonate) in which the nucleating metals are less soluble, and(ii) metal cation concentrations which are in equilibria balance withoxides; an increase in one, diminishes the solubility of the other, andtherefore addition of oxide to the carbonate electrolyte will diminishthe solubility and availability of the transition metal nucleatingagents.

Other physical chemical conditions to favor layered graphenemorphologies over CNTs include: (i) a decrease in the electrolysistemperature, (ii) a decrease in the concentration of lithium in themolten carbonate electrolyte replaced by an increase in larger thanlithium species, and with decreased lithium concentration even at highertemperatures, and (iii) conditions of higher electrolysis currentdensity. Consistent with these observations are the mechanisticimplications inhibiting nucleation that a decrease in temperature willdecrease the rate of carbonate mass transport to a point source fornucleation, which will have a greater inhibiting effect than a wide areadiffusion to a growing nano carbon structure (i.e., less material isprovided for reduction and carbon growth). A larger cation than lithiumwill face a larger energy barrier when attempting to permeate thenucleation site and growing CNT walls to provide needed chargecompensation during the ongoing growth process. Similarly, the greatermass transport required at higher current density will favor thetwo-dimensional diffusion consistent with the scaffold's largely planargrowth, rather than the point source diffusion consistent with anucleation point growth process. Each of these techniques can be usedalone or in any combination to inhibit, suppress, or prevent transitionmetal nucleation.

According, in one aspect, the present invention relates to a method forproducing a graphene carbon nanomaterial. In one embodiment, the methodcomprises:

-   -   (i) performing electrolysis between an electrolysis anode and an        electrolysis cathode in a molten carbonate electrolyte to        generate carbon nanomaterial on the cathode; and    -   (ii) electrochemically exfoliating the carbon nanomaterial (for        example, from a second anode) to produce graphene.

In one embodiment of any of the methods described herein, step (i) isperformed without a transition metal on or adjacent to the surface ofthe cathode.

In one embodiment of any of the methods described herein, theelectrolysis anode and molten carbonate electrolyte in step (i) do notinclude a transition metal. In another embodiment of any of the methodsdescribed herein, the electrolysis anode, electrolysis cathode, andmolten carbonate electrolyte in step (i) do not include a transitionmetal that is molten above the electrolyte melting point, such as zinc,tin, lead, cadmium, mercury or aluminium.

In another embodiment, the electrolysis is performed in the absence ofan oxide, such as an alkali metal oxide (e.g., lithium oxide).

In one embodiment of any of the methods described herein, theelectrolysis in step (i) is performed in the absence of a transitionmetal. In one embodiment of any of the methods described herein, theelectrolysis in step (i) is performed in the absence of a transitionmetal other than zinc.

In one embodiment of any of the methods described herein, step (i)comprises

-   -   (a) heating a carbonate electrolyte to obtain a molten carbonate        electrolyte;    -   (b) disposing the molten carbonate electrolyte between an        electrolysis anode and an electrolysis cathode in a cell; and    -   (c) applying an electrical current to the electrolysis cathode        and the electrolysis anode in the cell to electrolyze the        carbonate and generate carbon nanomaterial (e.g., carbon        nanoplatelets) on the electrolysis cathode.

In one embodiment of any of the methods described herein, step (ii)comprises performing electrolysis where the electrolysis cathode fromstep (i) having the carbon nanomaterial is used as an anode to producegraphene.

In one embodiment of any of the methods described herein, theelectrolysis cathode having the carbon nanomaterial is cooled prior toperforming the exfoliation.

In one embodiment of any of the methods described herein, step (ii)comprises (a) placing the cathode having carbon nanomaterial from step(i) from the electrolysis cathode as an exfoliation anode in anelectrochemical cell containing an exfoliation cathode and anexfoliation electrolyte, (b) applying an electrical voltage between theexfoliation anode and the exfoliation cathode to exfoliate graphene fromthe exfoliation anode, and (c) optionally, collecting grapheneexfoliated from the exfoliation anode.

In one embodiment of any of the methods described herein, theelectrolyzed carbonate in step (i) is replenished by addition of carbondioxide.

In one embodiment of any of the methods described herein, the source ofthe added carbon dioxide is one of air, pressurized CO₂, concentratedCO₂, a power generating industrial process, an iron generatingindustrial process, a steel generating industrial process, a cementformation process, an ammonia formation industrial process, an aluminumformation industrial process, a manufacturing process, an oven, asmokestack, or an internal combustion engines.

In one embodiment of any of the methods and systems described herein,the electrolysis cathode comprises stainless steel, cast iron, a nickelalloy such as, but not limited to, C276 (UNS N10276—anickel-molybdenum-chromium alloy containing tungsten), Inconel®(nickel-chromium based superalloys) (available from Special Metals Co.of New Hartford, N.Y., USA) or Nichrome (nickel-chrome alloy), or amaterial that resists corrosion in the presence of the molten carbonateelectrolyte, such as, for example, alumina ceramic, or any combinationof the foregoing.

In one embodiment of any of the methods and systems described herein,the electrolysis anode comprises iridium, platinum, a material that iselectrocatalytically active towards carbonate oxidation while resistingcorrosion in the presence of the molten carbonate electrolyte, or anycombination of the foregoing.

In one embodiment of any of the methods described herein, theelectrolysis cathode is coated with zinc, e.g., stainless steel coatedwith zinc.

In one embodiment of any of the methods described herein, in step (i),electrical current is applied with stepwise increases, or any othermanner of gradual current increases. For example, in certain embodimentsof any of the methods described herein, the electrolysis current isapplied for about 3 to about 30 minutes first at, for example, about0.01, then at about 0.02, then at about 0.04, then at about 0.08 A/cm²,followed by a longer duration, higher constant current density, such as,e.g., about 0.1, about 0.2 or about 0.5 A/cm².

In another embodiment of any of the methods described herein, the carbonnanomaterial growth comprises carbon nanoplatelets.

In another embodiment of any of the methods described herein, the carbonnanoplatelets comprise less than about 125 graphene layers, such as lessthan about 100 graphene layers, less than about 75 graphene layers, lessthan about 50 graphene layers, less than about 25 graphene layers, lessthan about 10 graphene layers or less than about 5 graphene layers.

In another embodiment of any of the methods described herein, thecarbonate electrolyte comprises an alkali metal carbonate, an alkaliearth metal carbonate, or any combination thereof.

In another embodiment of any of the methods described herein, the alkalimetal carbonate or alkali earth metal carbonate is lithium carbonate,sodium carbonate, potassium carbonate, rubidium carbonate, cesiumcarbonate, francium carbonate, beryllium carbonate, magnesium carbonate,calcium carbonate, strontium carbonate, barium carbonate, radiumcarbonate, or any mixture thereof.

In one embodiment of any of the methods described herein, the moltencarbonate electrolyte comprises lithium carbonate. In another embodimentof any of the steps for producing nano-materials, such asnano-platelets, described herein, the molten carbonate electrolytecomprises at least about 70, 80, 90, 95, 98, 99, or 100% of lithiumcarbonate, based upon 100% total weight of carbonate salts in theelectrolyte.

In another embodiment of any of the methods described herein, the moltencarbonate electrolyte further comprises one or more oxides, and/or oneor more oxygen, sulfur, halide, nitrogen or phosphorous containinginorganic salts.

In another embodiment of any of the methods described herein, step (ii)is performed in the presence of an exfoliation electrolyte, and theexfoliation electrolyte comprises an aqueous solution.

In another embodiment of any of the methods described herein, theexfoliation electrolyte comprises an aqueous solution of ammoniumsulfate.

In another embodiment of any of the methods described herein, theexfoliation electrolyte comprises a nonaqueous solution, such as forexample, a chlorinated hydrocarbon, such as, e.g., chloroform, or analcohol, such as, e.g., isopropanol, or any combination thereof.

In another embodiment of any of the methods described herein, theexfoliation electrolyte further comprises a carbonate dissolvingsolution.

In another embodiment of any of the methods described herein, theexfoliation is performed by electrolysis between an exfoliation anodeand the exfoliation cathode in an exfoliation electrolyte, where theexfoliation anode and the exfoliation cathode are separated by amembrane, filter, diaphragm or porous separator to isolate the grapheneproduced within the vicinity of the anode.

In another embodiment of any of the methods described herein, thegraphene produced comprises less than 10 graphene layers, such as lessthan 5 graphene layers. In another embodiment of any of the methodsdescribed herein, the graphene produced comprises a single layer ofgraphene.

In another embodiment of any of the methods described herein, thecoulombic efficiency of the process described in step (i) of anyembodiment herein is greater than about 80%, such as greater than about85%, greater than about 90%, or greater than about 95%. In anotherembodiment of any of the methods described herein, the coulombicefficiency of the process described in step (i) of any embodiment hereinis about 100%.

In another embodiment of any of the methods described herein, theelectrolysis reaction described in step (i) of an embodiment herein isperformed at a current density of between about 5 and about 5000 mA cm²,such as between about 50 and about 1000 mA cm², or between about 100 andabout 600 mA cm².

In another embodiment of any of the methods described herein, thegraphene carbon nanomaterial has a purity greater than about 80%, suchas greater than about 85%, greater than about 90%, greater than about95%, greater than about 97.5% or greater than about 99%.

In another embodiment of any of the methods described herein, thegraphene carbon nanomaterial exhibits a 2D peak in the Raman spectrum atless than 2720 cm⁻¹. In another embodiment of any of the methodsdescribed herein, the graphene carbon nanomaterial exhibits a 2D peak inthe Raman spectrum between 2679 and 2698 cm⁻¹. In yet another embodimentof any of the methods described herein, the graphene carbon nanomaterialexhibits a 2D peak in the Raman spectrum at 2679 cm⁻¹.

One embodiment is a method of forming graphene carbon nanomaterialcomprising (i) heating a carbonate electrolyte to obtain a moltencarbonate electrolyte; (ii) disposing the molten carbonate electrolytebetween an electrolysis anode and an electrolysis cathode in a cell;(iii) applying an electrical current to the electrolysis cathode and theelectrolysis anode in the cell to electrolyze the carbonate and producecarbon nanomaterial on the electrolysis cathode, wherein theelectrolyzed carbonate is replenished by addition of carbon dioxide;(iv) placing the electrolysis cathode on which carbon nanomaterial hasformed as an exfoliation anode in an electrochemical cell containing anexfoliation cathode and an exfoliation electrolyte; (v) applying anelectrical voltage between the exfoliation anode and (vi) theexfoliation cathode to exfoliate graphene from the exfoliation anode;and optionally collecting graphene exfoliated from the cathode of thecell. In one embodiment, the electrolysis in step (iii) is performed inthe absence of an oxide, such as an alkali metal oxide (e.g., lithiumoxide).

Another embodiment refers to a system to produce graphene carbonnanomaterial, the system comprising:

-   -   a furnace chamber to accept carbonate, the furnace chamber being        heated to produce molten carbonate; and    -   an electrolysis device having an anode and a cathode to apply        electrolysis to the molten carbonate,        wherein the system is configured to (i) initially form carbon        nanoplatelets on the cathode of the electrolysis device,        which (ii) subsequently are used as the anode in an        electrochemical exfoliation process to produce graphene carbon        nanomaterial. In one embodiment, the carbon nanoplatelets are        formed on the cathode in the absence of an oxide, such as an        alkali metal oxide (e.g., lithium oxide).

Another embodiment relates to a method for producing carbonnano-platelets (e.g., a two dimensional layered graphene product)comprising:

-   -   (a) heating a carbonate electrolyte to obtain a molten carbonate        electrolyte, wherein the molten carbonate may optionally further        comprise a metal (such as zinc) which is molten at the        temperature at which electrolysis is performed step (c);    -   (b) disposing the molten carbonate electrolyte between an        electrolysis anode and an electrolysis cathode in a cell; and    -   (c) applying an electrical current to the electrolysis cathode        and the electrolysis anode in the cell to electrolyze the        carbonate and generate carbon nano-platelets on the electrolysis        cathode, without the formation of transition metal nucleation        sites on the cathode. The formation of transition metal        nucleation sites may be inhibited, suppressed or prevented by        any of the techniques described herein.

In one embodiment, the electrolysis anode and the molten carbonateelectrolyte do not include a transition metal nucleating agent (e.g.,the electrolyte and anode do not release transition metal agents whichfacilitate nucleation of carbon on the cathode).

In another embodiment, the cathode (prior to and/or during the reactionprovided by step (c)) also does not include a transition metalnucleating agent. In yet another embodiment, the cathode includes one ormore transition metals, but the transition metals do not facilitate theformation of nucleation sites for carbon product formation in step (c)(for example by adding an oxide to decrease the solubility of thetransition metals in the electrolyte and at or near the cathode). Inanother embodiment, the method further includes electrochemicallyexfoliating the carbon nano-platelets (for example, from a second anode)to produce graphene.

Another embodiment relates to a method for producing carbon-onions(e.g., a three-dimensional concentric spherical layered grapheneproduct) comprising:

-   -   (a) heating a carbonate electrolyte comprising an oxide additive        (e.g., an alkali metal oxide such as lithium oxide) to obtain a        molten carbonate electrolyte;    -   (b) disposing the molten carbonate electrolyte between an        electrolysis anode and an electrolysis cathode in a cell,        wherein the electrolysis anode and the molten carbonate        electrolyte do not include a transition metal nucleating agent;    -   (c) applying an electrical current to the electrolysis cathode        and the electrolysis anode in the cell to electrolyze the        carbonate and generate carbon nano-onions on the electrolysis        cathode, without the formation of transition metal nucleation        sites on the cathode.

In one embodiment, the electrolysis anode and the molten carbonateelectrolyte do not include a transition metal nucleating agent (e.g.,the electrolyte and anode do not release transition metal agents whichfacilitate nucleation of carbon on the cathode).

In another embodiment, the cathode (prior to and/or during the reactionprovided by step (c)) also does not include a transition metalnucleating agent. In yet another embodiment, the cathode includes one ormore transition metals, but the transition metals do not facilitate theformation of nucleation sites for carbon product formation in step (c)(for example by adding an oxide to decrease the solubility of thetransition metals in the electrolyte and at or near the cathode).

In another embodiment, the method further includes electrochemicallyexfoliating the carbon nano-onions (for example, from a second anode) toproduce graphene.

Another embodiment is a system to produce carbon nano-onions (e.g., athree-dimensional concentric spherical layered graphene product)comprising:

-   -   a furnace chamber to accept carbonate, the furnace chamber being        heated to produce molten carbonate which comprises an oxide        additive (e.g., an alkali metal oxide, such as lithium oxide);        and    -   an electrolysis device having an anode and a cathode to apply        electrolysis to the molten carbonate,        wherein the system is configured to form carbon nano-onions on        the cathode of the electrolysis device, without the formation of        transition metal nucleation sites on the cathode.

In one embodiment, the anode and the molten carbonate electrolyte do notinclude a transition metal nucleating agent (e.g., the electrolyte andanode do not release transition metal agents which facilitate nucleationof carbon on the cathode).

In another embodiment, the cathode (prior to and/or during the reactionprovided by step (c)) also does not include a transition metalnucleating agent.

In yet another embodiment, the cathode includes one or more transitionmetals, but the transition metals do not facilitate the formation ofnucleation sites for carbon product formation during electrolysis.

In one embodiment, the system is further configured to subject thecarbon nano-onions to an electrochemical exfoliation process to producegraphene carbon nanomaterial (for example, by subsequently using thecathode of the electrolysis device as the anode in the electrochemicalexfoliation process).

Another embodiment relates to a method for producing carbon nano-onions(e.g., a three-dimensional concentric spherical layered grapheneproduct) comprising:

-   -   (a) heating a carbonate electrolyte comprising an oxide additive        to obtain a freshly melted carbonate electrolyte;    -   (b) disposing the freshly melted carbonate electrolyte between        an electrolysis anode and an electrolysis cathode in a cell,        wherein the electrolysis anode and/or the molten carbonate        electrolyte optionally further comprises a transition metal        nucleation agent;    -   (c) applying an electrical current to the electrolysis cathode        and the electrolysis anode in the cell to electrolyze the        freshly melted carbonate and generate carbon nano-onions on the        electrolysis cathode (e.g., without the formation of transition        metal nucleation sites on the cathode).

In one embodiment of the methods described herein for producingnano-onions, a constant current is applied during the electrolysis.

In another embodiment of the methods described herein for producingnano-onions, the method further includes electrochemically exfoliatingthe carbon nano-onions (for example, from a second anode) to producegraphene.

Another embodiment relates to a method for producing graphene carbonnano-scaffolds, which may be achieved by, e.g., suppressing theconcentration of lithium in the electrolyte, such as by replacing aportion of the lithium carbonate with a non-lithium carbonate,containing a larger than lithium cation (e.g., sodium or potassium), andsimultaneously inhibiting the formation of transition metal nucleationsites on the cathode comprising:

-   -   (a) heating a carbonate salt to obtain a molten carbonate        electrolyte enriched in non-lithium salts;    -   (b) disposing the molten carbonate electrolyte between an        electrolysis anode and an electrolysis cathode in a cell,        wherein the electrolysis anode and/or the molten carbonate        electrolyte optionally further comprises a transition metal        nucleation agent; and    -   (c) applying an electrical current to the electrolysis cathode        and the electrolysis anode in the cell to electrolyze the        carbonate and generate carbon nano-scaffolds, wherein if a        transition metal nucleation agent is present, inhibiting        activation of the transition metal nucleation agent during step        (c).

In one embodiment, the suppression of the lithium salt in theelectrolyte is achieved by conducting the process in an electrolytecomprising a carbonate salt containing less than about 50%, 60%, 70%,75%, 80%, 90%, or 100% lithium carbonate and enriched in non-lithiumcarbonates (e.g., Na₂CO₃ or K₂CO₃, or a combination thereof), based upon100% total weight of carbonate salts in the electrolyte. For instance,the electrolyte may comprise from about 10, 20, 30, 40, 50, 60, 70, 80,or 90% lithium carbonate, based upon 100% total weight of carbonatesalts in the electrolyte. The electrolyte may contain from about 10, 20,30, 40, 50, 60, 70, 80, or 90% of a non-lithium salt (such as Na₂CO₃ orK₂CO₃, or a combination thereof), based upon 100% total weight ofcarbonate salts in the electrolyte.

In one embodiment, formation of transition metal nucleation sites isinhibited by conducting step (c) at a temperature less than about 700°C.

In one embodiment of the above methods and systems, the cathode (priorto and/or during the reaction provided by step (c)) also does notinclude a transition metal nucleating agent. In another embodiment, thecathode includes one or more transition metals, but the transitionmetals do not facilitate the formation of nucleation sites for carbonproduct formation in step (c). In a further embodiment, the electrolysisanode and the molten carbonate electrolyte do not include a transitionmetal nucleating agent. In another further embodiment, the electrolysisis conducted at high current density, such as at least 0.4 A cm⁻² orhigher to inhibit formation of transition metal nucleation sites.

Other aspects and features of the present invention will become apparentto those of ordinary skill in the art upon review of the followingdescription of specific embodiments of the invention in conjunction withthe accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

In the figures, which illustrate, by way of example only, embodiments ofthe present invention:

FIG. 1 depicts an exemplary illustration of the method ofelectrosynthesis of graphene from CO₂. In FIG. 1A, CO₂ from the air orflue gas is electrolytically split to carbon nanoplatelets by moltencarbon electrolysis. In FIG. 1B, the carbonate synthesis cathode isplaced in a cellulose tube containing e.g., aqueous (NH₄)₂SO₄. In FIG.1C, the cellulose tube is placed in an (NH₄)₂SO₄ bath and exfoliated.

FIG. 2 is a scanning electron microscopy (SEM) image of the electrolysisproduct formed by splitting CO₂ in molten carbonate in the absence ofnickel nucleation and in the presence of zinc. FIG. 2 shows theformation of carbon platelets.

FIG. 3 shows photographs (FIGS. 3A and 3C, electrodes before and afterelectrolysis, respectively), SEM (FIGS. 3D and 3E), Raman spectroscopy(FIG. 3F) and X-ray diffraction (XRD) (FIG. 3G) of the electrolysisproduct formed by splitting CO₂ in molten carbonate, using a zinc coatedstainless steel cathode, illustrating electrosynthesis of carbonplatelets from CO₂. FIG. 3B depicts the measured cell potential duringelectrolysis.

FIG. 4 shows the solubility of Li₂CO₃ in water (top graph) and variousaqueous (NH₄)₂SO₄ solutions (bottom graph) as a function of temperature.

FIG. 5 shows tunneling electron microscopy (TEM) (FIGS. 5A and 5B, priorto exfoliation and subsequent to exfoliation, respectively), Ramanspectroscopy (FIG. 5C) and atomic force microscopy (ASM) images (FIG.5D) of a graphene product prepared according to the present invention.

FIG. 6 is a SEM image of the electrolysis product formed by splittingCO₂ in molten carbonate in the absence of in the absence of a transitionmetal nucleating agents and in the presence of lithium oxide. FIG. 6shows the formation of carbon nano-onions.

FIG. 7 shows TEM images showing a carbon nano-onion product. The bottomof FIG. 7 shows the interspatial graphene layer between the individualCNT walls in the adjacent SEM, and that the distance between 8 walls is2.841 nm amounting to 0.255 nm between layers.

FIG. 8 shows SEM images showing a carbon nano-onion product subsequentto extended duration electrolysis.

FIG. 9 shows SEM images of an electrolysis product produced in variouspure or mixed electrolytes.

FIG. 10 shows the formation of carbon nano-scaffolds. FIG. 10A shows ascheme of an electrolysis cell. FIG. 10B shows the electrolysiselectrodes before and after the electrolysis. FIGS. 10C1-10C6 show SEMimages of the electrolytic product produced under conditions of adecrease in electrolysis temperature and a decrease in concentration oflithium carbonate.

FIG. 11 shows SEM images of the electrolytic product produced under highor low current density conditions in various binary carbonateelectrolytes at various temperatures.

DETAILED DESCRIPTION OF THE INVENTION

It will be understood that any range of values described herein isintended to specifically include any intermediate value or sub-rangewithin the given range, and all such intermediate values and sub-rangesare individually and specifically disclosed.

It will also be understood that the word “a” or “an” is intended to mean“one or more” or “at least one”, and any singular form is intended toinclude plurals herein.

It will be further understood that the term “comprise,” including anyvariation thereof, is intended to be open-ended and means “include, butnot limited to,” unless otherwise specifically indicated to thecontrary.

When a list of items is given herein with an “or” before the last item,any one of the listed items or any suitable combination of two or moreof the listed items may be selected and used.

The term “nanomaterial” generally refers to a material (i) having atleast one limiting dimension of size less than 1000 nm, but otherdimensions in the material can be larger (for example, carbon nanotubeswith length much longer than 1000 nanometers are still carbonnanomaterials when their diameter (rather than their length) is lessthan 1000 nanometers), (ii) where the structure of the material may benanometer dimension building blocks (e.g., many layers of graphene)repeated to a greater than 1000 nm size, or (iii) composed of wallswhich have a nanoscopic thickness (even if the diameter of the materialis greater than 1000 nanometers).

The processes described herein include the synthesis of carbonnanomaterials and their subsequent conversion to graphene.

The present process splits carbon dioxide by electrolysis in moltencarbonate. Isotopic¹³C tracking may be used to follow the consumption ofCO₂, as it is dissolved in molten carbonate and is split by electrolysisto form carbon nanomaterials, such as carbon nanoplatelets. CO₂dissolution in molten lithium carbonate is exothermic and rapid, whichalong with heat generated by the electrolysis provides thermal balanceduring carbon deposition on the cathode. The process (in the absence ofa transition metal nucleating agent) where electrolysis is performedwith lithium carbonate forms carbon nanomaterials (CNM), oxygen anddissolved lithium oxide:Electrolysis:Li₂CO₃→C_(CNM)+O₂+Li₂O  (1a)

The electrolyte used in the electrolysis step to produce the carbonnanomaterials may be pure lithium carbonate (Li₂CO₃) or may containlithium carbon with one or more of added oxides, added sodium, calcium,or barium carbonates, or added boron, sulfur, phosphorus or nitrogendopants, or any combination of any of the foregoing. CO₂ added to theelectrolyte dissolves and chemically reacts with lithium oxide to renewand reform Li₂CO₃:Chemical Dissolution:CO₂+Li₂O→Li₂CO₃  (2)

In the processes described herein, carbon nanomaterials, such as carbonnanoplatelets, are formed by molten carbonate electrolysis whentransition metal nucleating agents (e.g., transition metals other thanzinc) are excluded. The processes described herein may be facilitated byincreasing the electrolysis current in a step-wise manner prior to theconstant current electrolysis:Electrolysis:Li₂CO₃→C_(platelets)+O₂+Li₂O  (1b)

In one embodiment, to avoid formation of carbon nanotubes (CNT), theelectrolyte and cathode surface are substantially free or free oftransition metal nucleating agents, such as nickel or chromium, whichcan nucleate CNT formation.

The carbon nanomaterials, such as carbon platelets, are then convertedto graphene by exfoliation:Exfoliation(DC voltage):C_(platelets)→C_(graphene)  (3)

In addition to carbon nanomaterial (such as carbon platelet) formation,the second product of molten carbonate CO₂ electrolysis in Equation 3 isthe evolution of pure oxygen, O₂, during the electrolysis. Asillustrated in FIG. 1 , the net reaction of Equations 1b, 2 and 3 is CO₂split by electrolysis into graphene and oxygen:CO₂→C_(graphene)+O₂  (4)

CO₂ electrolysis in molten carbonate production of carbon nanomaterialsreadily scales upward linearly with the area of the electrolysiselectrodes, facilitating larger scale synthesis of graphene. The moltencarbonate carbon nanomaterial electrolysis anode is not consumed andemits oxygen. The molten carbonate electrolysis does not consume carbonas a reactant and uses a no-cost oxide as the reactant to be reduced.

The carbon nanomaterial product resides on the cathode, which thereforemay be stacked vertically in a low physical footprint configuration. Thecarbon nanomaterial molten carbonate electrolysis process can operateunder relatively mild conditions (such as 770° C.) in a molten carbonateelectrolyte at 0.8 to 2 V potential. The electricity costs per tonne areestimated as $360 compared to the known costs of $602 per tonne foraluminum. These inexpensive costs provide a significant incentive to usethe greenhouse gas carbon dioxide as a reactant to produce graphene. Theprocesses described herein provide a useful path forward to help breakthe anthropogenic carbon cycle to mitigate climate change.

EXAMPLES Example I

Small transition metal clusters, including nickel, chromium and others,act as nucleation points to facilitate high yield C2CNT carbon nanotubegrowth. Zinc, although liquid at molten carbonate temperatures, lowersthe energy of the initial carbon deposition. In the absence of a solidtransition metal as nucleating agent (nucleating point), galvanized(zinc coated) steel was still shown to be an effective cathode forcarbon growth, but CNTs were scarce, comprising <1% of the carbonproduct. Instead the product, as shown in FIG. 2 , is an impure mix ofultra-thin carbon platelets, other carbon nanostructures and amorphouscarbon. FIG. 2 shows an SEM image of the washed cathode product from anickel free, 90 minute, 1 A constant current electrolysis in 730° C.molten Li₂CO₃ with 6 m (6 moles/kg Li₂CO₃) Li₂O (Alfa Aesar 99.5%). Theelectrolysis used a 5 cm² Pt foil anode and a 5 cm² 0.12 cm diametercoiled galvanized steel wire cathode.

The noble iridium/platinum anode utilized in this example was purposelyselected to inhibit carbon nanotube (CNT) formation. This enhances theobserved formation of the desired graphene product by preventingintroduction from the anode, migration, reduction and formation ofnickel or chromium nucleation sites on the cathode that favor formationof alternative CNT products. However, an iridium, platinum or iridiumalloy anode is not a prerequisite for high yield platelet or graphenegrowth. The inhibition of low levels of nickel migration from a nickelor nickel containing alloy anode or use of a thin film (e.g., betweenabout 10 and about 100,000 nm thick, such as between about 50 and about10,000 nm thick, or between about 100 and about nm thick) iridium anodeis viable. The following references describe thin film iridiumdeposition: Grushina et al., J. Appl. Chem. USSR, 2015, 1992, 65;Kamegaya et al., Electrochimica Acta, 1995, 40, 889; Ohsaka et al., Int.J. Surface Eng. Coatings 2007, 85, 260; Ohsaka et al., Electrochem,Solid-State Lett., 2010, 13, D65; Shuxin et al., Rare Metal Mat. Eng.,2015, 44, 1816; Lopez et al., Int. J. Electrochem. Sci., 2015, 10, 9933;Allahyarzadeh et al., Surface Rev. Lett., 2016, 23, 1630001; and Sheelaet al., Int. J. Surface Eng. Coatings, 2017, 8:5, 191.

A mixture of nanostructures including a large fraction of plateletsforms during the first few minutes (e.g., 5 minutes) of electrolysis,even in the presence of nickel. However, in the presence of nickel withextended electrolysis time (such as, e.g., 15 minutes), the productquickly resolves into carbon nanotubes. This is the case with a widerange of lithiated electrolytes, using a wide range of metal cathodes,including galvanized steel and copper, and over a range of electrolysistemperatures from 730 to 790° C. Higher temperatures, which were notused in this study, increasingly favor the two electron reduction of CO₂to CO, and by 950° C. the product is pure carbon monoxide.

Example II

In this example, it is shown that performing the electrolysis in theabsence of other transition metal nucleating agents, but in the presenceof zinc, carbon nano platelets, rather than carbon nano-onions (CNOs) orcarbon nanotubes (CNTs), form. Zinc is present as the surface coating onthe (galvanized) steel cathode. The yield of carbon platelets observedin FIG. 2 increases to 70% when the electrolyte is pure Li₂CO₃ ratherthan 6 m (6 molal) Li₂O, and to over 95% when increasing constantcurrent steps (FIG. 3B) are first applied prior to the constant current.Specifically, in this electrolysis, graphite platelets are grown on a 5cm² galvanized (zinc coated) steel cathode with a 5 cm² Pt Ir foil anodein 770° C. Li₂CO₃ when the electrolysis current is increased stepwisefor 10 min. at 0.05 and 0.10 A, then 5 min. at 0.2 and 0.4 A followed bya constant of 1 A for 2 hours. These experimental conditions (zinc onthe cathode, pure Li₂CO₃ electrolyte, neither Ni nor Cr in the anode,and increasing constant current steps) were chosen to increase the yieldof the carbon platelets. Replicate experiments produced similar resultsof over 95% carbon platelets yield. The 2-hour constant currentelectrolysis occurs at 0.2 A cm⁻², consuming during the 2-hourelectrolysis 0.82 g CO₂ and producing 0.21 g carbon platelets. Thepotential of the stepped current electrolysis and the electrolysisproduct are presented in FIG. 4B. The product purity is over 95%. Theremainder includes smaller particles, which also contain smallerplatelets. X-ray diffraction (XRD) of the product (FIG. 3G) exhibits asharp peak at 26.3° 20, indicative of a high degree of graphiticallotrope crystallinity. Raman spectroscopy (FIG. 3F) and TEM (FIG. 3A),indicates the platelets have a relatively low number (25 to 125)graphene layers. Without wishing to be bound by theory, the inventortheorizes that by starting with fewer graphene layers compared tographite, these ultrathin platelets electrochemically exfoliate to ahigher quality (thinner) graphene for an overall production of graphenefrom CO₂ by electrolysis and electrochemical exfoliation, in accordancewith Equation 5.

An important feature for the conversion of graphite to graphene is a redshift in the Raman spectrum 2D peak compared with graphite (2720 cm⁻¹)(see, e.g., Zhou et al., Mat. Lett., 2019, 235, 153). The 2D-band ishighly sensitive to the number of graphene layers, with single layerexhibiting a peak at 2679 cm⁻¹, and 1-4 layers exhibiting a peak at 2698cm⁻¹. Even prior to electrochemical exfoliation, the ultrathin carbonplatelets produced by molten carbonate synthesis (FIG. 3F) exhibit asignificant red shift to 2708 cm⁻¹. In FIG. 3F, the intensity ratioI_(D)/I_(D′) is 1.3, demonstrating that for the whole range ofI_(D/)I_(D′), the defect level is always below the benchmark forgraphene boundary defects (I_(D)/I_(D′)=3.5). (The ratio I_(D)/I_(D′)represents the intensity ratio for the D peak (1350 cm⁻¹) and D′ peak(1620 cm⁻¹).) The ratio of Raman D or 2D to the G peaks are respectivelyassociated with the number of defects and degree of graphitization. InFIG. 3F, the intensity ratio of the Raman I_(D)/I_(G) peak is a low(0.4), and that of Raman I_(2D)/I_(G) is 0.6, which both indicate asmall quantity of defects. (The ratio I_(D)/I_(G) represents theintensity ratio for the D peak (1350 cm⁻¹) and G peak (1583 cm⁻¹).)

Example III

In this example, it is shown that lithium carbonate entrapped with thecarbon platelets produced during the electrolysis described in ExampleII can be readily removed by dissolution in aqueous ammonium sulfatesolutions.

Unlike Na₂CO₃ and K₂CO₃ which are highly soluble in water, Li₂CO₃ has alow solubility (30.6, 113 and 1.2 g per 100 g H₂O, respectively, at 25°C.). Aqueous ammonium sulfate is one of the few media in which Li₂CO₃solubility is enhanced.

An aqueous medium was investigated capable of both sustainingexfoliation and conducive to the dissolution of excess lithium carbonateelectrolyte that congealed on the cathode during the molten lithiumcarbon electrolytic production and extraction and cooling of the cathodecontaining the carbon product. These solubility measurements aresummarized below. Solubility is measured both by incremental addition oflithium carbonate (Alfa Aesar) to water, or ammonium sulfate (AlfaAesar) in water until observation of excess lithium carbonate, and bydilution of excess lithium carbonate until observation of completedissolution.

Interestingly, whereas the aqueous solubility of sodium and potassiumcarbonate are high (30.6, and 113 per 100 g H₂O respectively at 25° C.)and increase with temperature (43.9/46, and 140/156 g H₂O, respectively,at 80/100° C.), the measured aqueous solubility of lithium carbonate islow and decreases with increasing temperature, as shown in the top traceof FIG. 4 . The aqueous solubility of lithium carbonate (1.2 g per 100 gH₂O at 25° C.) is low compared to the aqueous solubilities of lithiumchloride and lithium bromide (18.0 g and 17.5 per 100 g H₂O respectivelyat 25° C.), and increases with temperature (to 112/128, and 245/266 gH₂O, respectively, at 80/100° C.).

Next, the dissolution of ammonium sulfate in water (without lithiumcarbonate) was verified both at room temperature and approaching thesolution boiling point. See Table 1. These measurements were conductedto verify dissolution, not to establish ammonium sulfate solubilitylimits, which are estimated at 15% to 20% higher than the observedmaximum dissolution at each temperature. The solubility, as measuredmass (grams), of lithium carbonate soluble in 100 ml of either 1.07,2.33, 4.06 or 6.64 molal (NH₄)₂SO₄ is presented in the lower trace ofFIG. 4 . The 100° C. and 108.9° C. data in the lower trace of FIG. 5 arethe measured solubility limits of lithium carbonate respectively in 6.45or 6.64 molal ammonium sulfate. Increasing concentrations of aqueousammonium sulfate considerably enhances lithium carbonate solubility.

TABLE 1 Dissolution of Aqueous Ammonium Sulfate Solutions as a Functionof Temperature (NH₄)₂SO₄ in water C C C (mol/ (mol/ (mol/ H₂O (NH₄)₂SO₄per L per kg per kg Temperature (g) (g) solution) solution) H₂O) 25° C.93.3 13.2 1 0.94 1.07 25° C. 85.7 26.4 2 1.78 2.33 25° C. 77.1 49.5 3.752.7 4.56 100° C. 77.1 65.6 / 3.48 6.45 (64.3; 65.2; 65.6) 108.5° C. 77.167.5 / 3.54 6.64 (66.2; 67.3; 67.5)

Example IV

In this example, it is shown that the carbon platelets formed in ExampleII are converted to graphene by electrochemical exfoliation.

Securing the electrochemical exfoliation electrode within a cellulosedialysis membrane can isolate the graphene product from the bulkelectrolyte. The electrode within a cellulose membrane assembly is usedas the anode in a two-compartment electrochemical cell, but rather thanusing graphite, using the cooled cathode, unwashed (carbon nanoplatelet)cathode in 0.1 M (NH₄)₂SO₄ as shown in FIG. 1C. Specifically, thecarbonate synthesis cathode containing product is cooled and placed in acellulose tube containing aqueous 0.1 M (NH₄)₂SO₄. The cellulose tube isan inexpensive premium commercial cellulose dialysis membrane, (see,e.g.,https://www.amazon.com/s?k=Premium-Dialysis-Tubing-Regenerated-Cellulose)listed as a cutoff of 12-14 kdals, equivalent to 1 to 2 nm pore size. Asshown in FIG. 1C, the cellulose tube is placed in an 0.1 M (NH₄)₂SO₄bath with a counter electrode. DC voltage is then applied that generatesgas bursts between the graphene layers, exfoliating the thin plateletsand producing graphene. As graphene layers are peeled, the cellulosetraps them within the anode compartment.

Before exfoliation, the platelets range from 25 to 125 graphene layersas measured by TEM (see, e.g., FIG. 5A). This is consistent with themeasured Raman spectrum 2D peak (graphite red shifted) at 2708 cm⁻¹.After exfoliation, the lateral dimensions of the exfoliated layers are 3to 8 μm, as measured by SEM (FIG. 5B). After exfoliation, the product isfiltered, rinsed and freeze dried to remove water, then analyzed by TEM,atomic force microscopy (AFM), and Raman spectroscopy. The exfoliationproduct yield is 83% by mass of the original carbon platelets. Theproduct yield would likely rise with longer exfoliation times (such asmore than 10 hours).

Raman spectra of sample carbon nano-platelets produced by the C2CNTtechnique is shown in FIG. 5C top trace and of a sample grapheneproduced the C2CNT technique in FIG. 5C bottom trace. The presence ofthe D′-band is indicative of the layered single and multiple (platelet)graphene layers, and the left shift of the 2-D band indicates the thingraphene layer.

An important feature for the conversion of graphite to graphene is a redshift in the Raman spectrum 2D peak compared with graphite (2720 cm⁻¹)(see, e.g., Zhou et al., Mat. Lett., 2019, 235, 153). The 2D-band ishighly sensitive to the number of graphene layers, with single layerexhibiting a peak at 2679 cm⁻¹, and 1-4 layers exhibiting a peak at 2698cm⁻¹. Even prior to electrochemical exfoliation, the ultrathin carbonplatelets produced by molten carbonate synthesis (FIG. 3F) exhibit asignificant red shift to 2708 cm⁻¹. In FIG. 3F, the intensity ratioI_(D)/I_(D′) is 1.3, demonstrating that for the whole range ofI_(D/)I_(D′), the defect level is always below the benchmark forgraphene boundary defects (I_(D)/I_(D′)=3.5). (The ratio I_(D)/I_(D′)represents the intensity ratio for the D peak (1350 cm⁻¹) and D′ peak(1620 cm⁻¹).) The ratio of Raman D or 2D to the G peaks are respectivelyassociated with the number of defects and degree of graphitization. InFIG. 3F, the intensity ratio of the Raman I_(D)/I_(G) peak is a low(0.4), and that of Raman I_(2D)/I_(G) is 0.6, which both indicate asmall quantity of defects. (The ratio I_(D)/I_(G) represents theintensity ratio for the D peak (1350 cm⁻¹) and G peak (1583 cm⁻¹).

Raman spectra of sample carbon nano-platelets produced by the processdescribed herein is shown in the FIG. 5C bottom and compared to theRaman spectra of the sample graphene produced the process describedherein in. The presence of the D′-band (1620 cm⁻¹) is indicative of thelayered single and multiple (platelet) graphene layers, and the leftshift of the 2-D band indicates the thin graphene layer Raman spectra ofsample carbon nano-platelets produced by the process described herein isshown in the FIG. 5C bottom and compared to the Raman spectra of thesample graphene produced the process described herein in. The presenceof the D′-band (1620 cm⁻¹) is indicative of the layered single andmultiple (platelet) graphene layers, and the left shift of the 2-D bandindicates the thin graphene layer.

In FIG. 5C, the Raman 2D peak exhibits a significant red shift from 2708cm⁻¹ to 2690 cm⁻¹ from platelets (pre-exfoliation) to graphene(post-exfoliation) product. Both the platelets (pre-exfoliation) andgraphene (post-exfoliation) are red shifted from graphite (2720 cm⁻¹).This shift to 2690 cm⁻¹ is indicative of graphene ranging from to 1 to 5graphene layers thick. Edge TEM cross section of the exfoliation productalso exhibits graphene ranging from 1 layer (shown in the inset to FIG.5B) to 5 layers thick. This is verified by AFM (see FIG. 5D). Dispersionof the graphene product for AFM characterization remains a challenge.Sonication and freeze drying effectively disperses the product, but isoverly aggressive and converts the graphene from a continuous flake to“swiss cheese” like, which has the benefit of providing extra locationsfor depth determination (see FIG. 5D). For comparison, using graphitefoil as the exfoliating reactant, rather than the molten carbonatesynthesized carbon nanoplatelets, in the same experimental configurationproduces multi-layered graphene that is approximately 5 fold thicker,and ranges from 6 to 25 graphene layers thick, that exhibits a Raman2D-band peak at ˜2703 cm⁻¹, rather than 2690 cm⁻¹ observed for thecarbon nanoplatelet exfoliated product of Example II.

It is expected that the graphene products prepared by the processesdescribed herein may provide improved structural materials. For example,it was observed that a key measurable characteristic correlated tostrength is a low defect ratio as measured by the ratio of the ordered(G peak (1583 cm⁻¹), reflecting the cylindrical planar sp² bondingamongst carbons) as compared to disorder (D peak (1350 cm⁻¹), reflectingthe out of plane sp^(a) tetrahedral bonding amongst carbons) in theRaman spectra.

Raman spectroscopy of the graphene products prepared according to theprocesses described herein indicates that the exfoliation productexhibits increased defects compared to thicker pre-exfoliation plateletsformed during electrolysis in molten carbonate, but that the defectlevel remains low and within tolerated levels for graphene. From FIG.6C, peak ratios for graphene are compared to ratios for the platelets:the I_(D)/I_(D′) is 1.5 (for the graphene product, compared to 1.3 forthe nano-platelets), again demonstrating that for the whole range ofI_(D)/I_(D′) the defect level is always below the benchmark for grapheneboundary defect ratio of I_(D)/I_(D)′=3.5. The intensity ratio of theRaman I_(D)/I_(G) peak is 0.64 (for the graphene product, compared to0.4 for the nano-platelets) and that of Raman I_(2D)/I_(G) is 0.70 (forthe graphene product, compared to 0.6 for the nano-platelets), whichboth indicate a small amount of defects.

The majority of the applied exfoliation voltage is lost throughresistance drop over the 0.1 M ammonium sulfate solution. This may beavoided by placing the electrodes closer together and/or higher ionicstrength to lower energy requirements. The temperature can be increasedand the cellulose membrane can also be modified to minimize the voltagedrop and also increase the sustainable current density (and rate ofexfoliation).

Example V

The processes and systems described herein can also be modified and usedto produce other carbon nanomaterials (CNMs), including graphene,nano-onions, nano-platelets, nano-scaffolds and helical carbonnanotubes. It is observed that each of these CNMs exhibit unusual andvaluable physical chemical properties, such as, for example, lubrication(nano-onions), batteries (graphene) and environmental sorbents (nanocarbon aerogels) prior to addition to structure materials, and enhancedproperties including improved electrical conductivity and sensingability for CNM-structural material composites. In each case, theproduct may be synthesized to a high coulombic efficiency of over 95%,and in most cases the product had a purity over 95%.

Example VI

In this example, it is shown that performing the electrolysis in theabsence of a nickel and the near exclusion of any other impurity leveltransition metal nucleating agents, and in the absence of a stepwisecurrent increase, but in the presence of lithium oxide, which can serveto decrease solubility of any impurity presence of other transitionmetals, results in the formation of another graphene based morphologyconsisting of concentric spherical layers of graphene and resulting in ahigh yield of carbon nano-onions (CNOs), rather than the carbon nanoplatelets comprising two planar layered graphene as observed in ExampleII. Zinc is present as the surface coating on the (galvanized) steelcathode. The yield of carbon nano-onions shown in FIG. 6 is over 95%.Applications for inexpensive CNOs include supercapacitors, batteryanodes, and solid-lubricants. The geologic (graphite-like durability)stability of graphene allotrope carbon materials may provide a long-termrepository to store atmospheric CO₂. SEM, EDS, and TEM characterizationprovides fundamental evidence of the high yield and purity of the CNOsynthesis. Specifically, in this electrolysis, highly uniform carbonspheroids are grown on a 5 cm² galvanized (zinc coated) steel cathodewith a 5 cm² Pt Ir foil anode in 770° C. Li₂CO₃ containing 5.9 molalLi₂O when the electrolysis current is held constant at 1 A (0.2 A/cm²)for 1.5 hours. As measured by EDS, the carbon content of the product isover 99%, the purity of carbon spheroids in the product is over 95%, andthe coulombic efficiency of the electrolysis is over 95%. FIG. 6A showsan SEM trace of the product following 5 minutes, 15 minutes or 90minutes of electrolysis. As can be seen, the distinct carbon spheroidshape is evident even with an electrolysis duration of 15 minutes orless. FIG. 6C presents an overview (lower magnification SEM) of thevarious syntheses presented (at higher magnification) in FIG. 6B. Ineach case, the product is highly uniform diameter carbon spheroids. Eachof the spheroids in FIG. 6B is in turn formed from clusters ofnano-onions.

FIG. 7 shows a TEM of the carbon nano-onion (CNO) product after 30minutes of electrolysis. The distinctive concentric, shell morphology ofcarbon nano-onions with a 0.35 nm interlayer separation typical oflayered graphitic structures is evident. Not shown in the figure is thatthe separated, as well as individual bundled nano-onions in thespheroids, have an increasing average diameter with increasingelectrolysis time, as measured by ImageJ SEM automated optical countingsoftware. Respectively after 5, 30, and 90 minutes of electrolysis, theindividual CNOs have an increasing diameter of 38±10 nm, 66±6 nm and96±2 nm, while the spheroids (bundled nano-onions) have a combinedten-fold higher respective diameter of 400, 600, and 900 nm, as seen inFIG. 6B. While the short duration (5 minutes) electrolysis formednano-onions have a distinctive size, unlike the longer durationsyntheses, the product after 5 minutes of electrolysis does not yetexhibit the distinctive concentric spherical graphene shells evident inFIG. 7 .

As seen in the SEM of FIG. 8A, extended electrolysis (15 hours, ratherthan 1.5 hours), at lower current density (0.1, rather than 0.2, A/cm²)produces more of the carbon nano-onion product, but not a significantlylarger size of the carbon nano-onion product.

The SEM traces shown in FIG. 8B depict the product of a procedure inwhich carbon nano-onions are formed even in the presence of a transitionmetal which has been inhibited from promoting carbon nucleation.Generally, in an aged lithium carbonate electrolyte a high purity,uniform CNT product is obtained during an electrolysis at a controlledtemperature in the 700° C. range, and the degree to which the carbonnanotube product is tangled or straight, long or short, or thick or thincan be controlled by additives to the lithium carbonate electrolyte,current density, electrolysis duration, and choice of anode or cathodematerial. However, when the electrolyte is not aged, the product can bea partial or pure carbon nano-onion product instead. Aging refers toallowing the electrolyte to sit in a molten state for a period ofseveral hours to several days prior to use. Subsequent to initiation ofan electrolysis in a freshly melted solution, it is observed there is atime, for example one hour, before CO₂ is fully absorbed in theelectrolyte. After that period, CO₂ is fully absorbed up to a rateequivalent to the 4 Faraday per mole CO₂ of the constant current appliedin the electrolysis. It is observed that the activation period for CO₂to be absorbed during the electrolysis start-up can be shortened by 2 to3-fold when Li₂O has been added to the lithium carbonate electrolyte.This period of time appears to correlate with the necessary time for themolten carbonate to achieve a steady state concentration of Li₂O, forexample in accord with the equilibrium reaction:Li₂CO₃

CO₂+Li₂O

Without wishing to be being bound by any theory, it is proposed thattransition metal nucleation of carbon nanotube growth is inhibitedduring this initiation period of electrolyte activation. Specifically,an electrolysis is conducted in freshly melted 770° C. molten Li₂CO₃using a Muntz brass cathode and Inconel 718 anode both with active areaof 2450 cm². The electrolysis is conducted at 0.2 A/cm² for a durationof 16 hours. As shown in FIG. 8B, the washed cathode product is purecarbon nano-onions without any evidence of carbon nano-tubes.

Example VII

In this example, it is shown that performing the electrolysis in a highconcentration sodium or potassium molten carbonate electrolyte forms analternative graphene product, carbon nano-scaffolds. Rather than a flat,multilayered graphene platelet morphology, carbon nano-scaffolds consistof a morphology in which multilayered graphene is stacked at sharpangles in an open structure, This open structure is not onlyaesthetically distinct, but exposes a larger surface area of graphene,which has the potential to increase activity in graphene capacitor,battery, EMF shielding and catalytic applications. Furthermore, theconditions of carbon nano-scaffold growth are distinctive from theplatelet growth conditions described above. Specifically, unlike theavoidance of transition metals to prevent competitive growth of analternative carbon nanotube product, here (i) transition metal ions arepermitted, for example as introduced by the anode, and the moltencarbonate CO₂ electrolysis is conducted in (ii) electrolytes and/or at(iii) temperature conditions that are specifically not conducive tocarbon nanotube (CNT) growth.

It has been shown (see, e.g., Wu et al., Carbon., 2016, 106, 208) thattemperatures greater than 700° C. are more conducive to CNT growthduring molten carbonate electrolysis. Here, it is also demonstrated thatelectrolytes with an increasing fraction of Na₂CO₃ or K₂CO₃ in a mixedLi₂CO₃ electrolysis are less conducive to CNT growth even in thepresence of nucleating transition metals. FIG. 9 shows SEM of theelectrolysis product in various mixed electrolytes compared to that inFIG. 9A conducted in a pure, 24 hour aged, 770° C. Li₂CO₃ electrolytesubsequent to a 5 hour electrolysis. Each of the electrolysis reactionswas conducted at a current density of 0.2 A/cm² with a cathode of MuntzBrass (an alloy of 60% Cu and 40% Zn) and an anode of Inconel 718 (analloy of 50-55% Ni, 17-21% Cr, 2, 4.75-5.5% Nb&Ta, 2.8-3.3% Mo, theremainder Fe and low concentrations of Ti, Co, Al, Mn, Cu, Si and C).The addition of 8% LiBO₂ to the electrolyte further improves themorphology, uniformity and purity of the carbon nanotube product. Forexample, addition of 8% LiBO₂ to the pure Li₂CO₃ increased the aspectratio (length to diameter) of the CNT product (not shown), and thisLiBO₂ was added to each of the mixed electrolytes to improve the lowerquality of the CNT product. (As discussed below, H₃BO₃ can be partiallyor completed substituted for LiBO₂ after water is allowed to leave thesystem.) The scale bars are 50 μm for FIGS. 9A and 9F, 20 μm for FIG.9D, and 10 μm for FIGS. 9B, 9C and 9E. As can be seen by comparing FIG.9A to FIG. 9F at the same scale, there are no CNTs readily observed inthe 60% Na₂CO₃/40% Li₂CO₃ electrolysis product, while the product ishighly pure CNTs in the 100% Li₂CO₃ electrolysis product. The 10% or 20%Na₂CO₃ electrolysis products contain over 90% CNT, while 30% Na₂CO₃ (notshown), and 50% Na₂CO₃ exhibit a diminishing yield of CNTs and anincreasing fraction of carbon nanospheres and carbon platelets. CNTaspect ratio decreases and the diameter increases with increasing Na₂CO₃percentage in the electrolyte (10% Na₂CO₃: ˜80 nm, 20% % Na₂CO₃: ˜100nm, 30% % Na₂CO₃: ˜200 nm, 50% % Na₂CO₃: ˜1 μm). For the 20 wt % K₂CO₃in Li₂CO₃, SEM shown FIG. 9D and 20 wt % K₂CO₃ (not shown) electrolysis,the loss of aspect ratio drop in CNT purity occurs more rapidly withincreasing K₂CO₃ weight fraction than the electrosynthesis withincreasing Na₂CO₃ fraction. Energy-dispersive X-ray spectroscopy (EDS)tests were employed to probe the elemental analysis of products from themixed electrolyte electrolysis. EDS of both the 20% Na₂CO₃ and 20% K₂CO₃samples are 100% carbon, while the 50% Na₂CO₃ and 50% K₂CO₃ spectra arerespectively 97.0% carbon (and 3.0% Na) and 97.8% carbon (and 2.2% K);boron in the CNTs is below the limits of EDS detection. The calculatedthermodynamic potential for the reduction of the alkali carbonatesincreases in the order E_(Li2CO3)<E_(Na2CO3)<E_(K2CO3). The highervoltage of an increasing concentration of the latter salts wouldincrease the possibility for reduction of the alkali cation to thealkali metal, rather than the desired reduction of carbonate to carbon.The coulombic efficiencies, comparing the mass of the product to theapplied 4e⁻ per mole of charge, approach 100% (98-100%) for the threecases of 100% Li₂CO₃, 10% Na₂CO₃, and 20% Na₂CO₃ electrolyteexperiments. Coulombic efficiency is still high, but decreased in binarylithium carbon electrolytes containing over 20% of sodium or potassiumcarbonate. For example, the coulombic electrolysis efficiency drops from95% for 30% Na₂CO₃ electrolyte to 93% 50% Na₂CO₃ electrolyte, and to 90%for the 60% Na₂CO₃ electrolyte. Carbonate electrolysis is decreasinglyconducive to a CNT product in electrolytes containing. >20 wt % Na₂CO₃or ≥20 wt % K₂CO₃.

In FIG. 10 is shown the distinctive carbon nano-scaffold product whenthe electrolysis is conducted at 670° C., rather than 770° C., in asimilar 50% Na₂CO₃/50% Li₂CO₃ electrolyte. While transition metalelements can again be release from the Inconel 718 anode, and while theMuntz brass cathode is comprised of copper and zinc, there is noevidence that the carbon nano-scaffold growth is based transition metalnucleation. In the electrolyte 10 wt % H₃BO₃, rather than LiBO₂, wasadded as a cost saving measure. H₃BO₃ can be partially or completedsubstituted for LiBO₂ after water is allowed to leave the system. Ascheme of the electrolysis cell is shown in FIG. 10A, and theelectrolysis electrodes before and after the electrolysis in FIG. 10B.SEM images of the product is shown in FIGS. 10C1-C6 with variousmagnifications. In total the electrolyte consisted of 250 g of Na₂CO₃,250 g of Li₂CO₃, and 50 g of H₃BO₃. The electrolysis was conducted at670° C. for 4.0 hours at a constant current of 5 A with 5 by 5 cmelectrodes. Voltage throughout the electrolysis was consistently 2.0 V,and over 85% of theoretically calculated CO₂ was converted to carbon.Over 80% of the product was the unusual carbon nano-scaffold morphology.The morphology consists of a series of asymmetric 50 to 200 nm thickflat multilayer graphene platelets 2 to 20 μm long oriented in a 3Dneoplasticism-like geometry.

FIG. 11 shows SEM of the electrolytic product produced under high(panels D through F) and low (panels) G or current density conditions invarious binary carbonate electrolytes at various temperatures. FIGS.11D-11F show carbon products with a higher current density (0.4 A/cm²),at a range of temperatures, with a different anode, Nichrome C (61% Ni,15% Cr, 24% Fe, the same cathode, and without any borate additive. Asseen in FIGS. 11D and 11E1, there is a significant carbon nano-scaffoldproduct even at the higher temperature of 750° C. At this temperatureand current density, the product of the 30% Na₂CO₃ electrolysis largeproportions of both carbon nano-scaffolds and carbon nano-onions. Notshown is that carbon nano-scaffolds are also observed in a 70 wt %Na₂CO₃ electrolyte, but the structures are smaller and are surrounded byamorphous carbon. At this temperature and current density, as seen inFIGS. 11E1 and 11E2, the product of the 30% K₂CO₃ electrolysis consistsmainly of carbon nano-scaffolds and ˜10% very thick carbon nanotubes.EDS verifies that the carbon nano-scaffold structures are largely carbon(98.3%) with a small amount of potassium (1.7%). The carbonnano-scaffold is observed at 50 wt % K₂CO₃ (not shown), but as seen inFIGS. 11F1-11F3, carbon nano-scaffolds are not observed in electrolyteswith high wt % of K₂CO₃ (70% K₂CO₃/30% Li₂CO₃) In this electrolyte, at570° C. the F1 panel product consists of small rounded, carbonassemblies, at 650° C. the F2 panel product consists of coral-likecarbon assemblies, and at 750° C. the F3 panel product consists oflarger, but less defined, coral-like carbon structures. Carbonateelectrolysis is conducive to a carbon nano-scaffold product inelectrolytes containing 30 to 70 wt % Na₂CO₃ or 30 to 50 wt % K₂CO₃ at650° C. or higher (e.g., 750° C. or higher). While transition metalelements can be included in the electrolysis system that produces thenano-scaffold product, there is no evidence that the carbonnano-scaffold growth is based on transition metal nucleation. The insetof panel 11D shows that with the high current density of 0.4 A/cm² in a60/40 wt % Na₂/Li₂CO₃ electrolyte, the nano-scaffold morphology is stillobserved when the temperature is decreased to 660° C. Carbonnano-scaffolds can also synthesized at a low current density of 0.1A/cm² when the temperature is decreased further to 570° C. as shown inFIG. 11 panel G, although the cross sectional width of each scaffoldunit is approximately 3-fold smaller than in FIG. C1 -C6 whensynthesized at high current density (0.4 A/cm²), higher temperature(670° C.) and with more lithium carbonate (50%) in the electrolyte.

Of course, the above described embodiments are intended to beillustrative only and in no way limiting. The described embodiments aresusceptible to many modifications of form, arrangement of parts, detailsand order of operation. The invention, rather, is intended to encompassall such modification within its scope, as defined by the claims.

What is claimed is:
 1. A method for producing carbon nano-onionscomprising: (a) heating a carbonate electrolyte comprising an oxideadditive to obtain a molten carbonate electrolyte; (b) disposing themolten carbonate electrolyte between an electrolysis anode and anelectrolysis cathode in a cell; (c) applying an electrical current tothe electrolysis cathode and the electrolysis anode in the cell toelectrolyze the carbonate and generate carbon nano-onions on theelectrolysis cathode without the formation of transition metalnucleation sites on the cathode.
 2. The method of claim 1, wherein theoxide additive comprises lithium oxide.
 3. The method of claim 1,wherein the molten carbonate electrolyte comprises one or moretransition metal nucleating agents and the conditions for electrolysisreduce the solubility of one or more of the transition metal nucleatingagents.
 4. The method of claim 3, wherein the transition metalnucleating agents for which solubility has been reduced are selectedfrom nickel, chromium, iron, and any combination of any of theforegoing.
 5. The method of claim 3, wherein the conditions for reducingthe solubility of one or more transition metal nucleating agents duringelectrolysis include (a) an electrolyte comprising (i) a lithiumcarbonate and (ii) one or both of sodium carbonate and potassiumcarbonate, (b) decreasing the electrolysis temperature, (c) decreasingthe concentration of lithium in the electrolyte, (d) increasing theelectrolysis current density, or (e) any combination of any of theforegoing.
 6. The method of claim 1, wherein the electrolyzed carbonatein step (c) is replenished by addition of carbon dioxide.
 7. The methodof claim 6, wherein the source of the added carbon dioxide is one ofair, pressurized CO₂, concentrated CO₂, a power generating industrialprocess, an iron generating industrial process, a steel generatingindustrial process, a cement formation process, an ammonia formationindustrial process, an aluminum formation industrial process, amanufacturing process, an oven, a smokestack, or an internal combustionengine.
 8. The method of claim 1, wherein the electrolysis cathodecomprises stainless steel, cast iron, a nickel alloy, a material thatresists corrosion in the presence of the molten carbonate electrolyte,or any combination of the foregoing.
 9. The method of claim 1, whereinthe electrolysis cathode is coated with zinc.
 10. The method of claim 1,wherein in step (c), electrical current is applied with stepwiseincreases.
 11. The method of claim 1, wherein the molten carbonateelectrolyte comprises an alkali metal carbonate, an alkali earth metalcarbonate, or any combination thereof.
 12. The method of claim 11,wherein the alkali metal carbonate or alkali earth metal carbonate islithium carbonate, sodium carbonate, potassium carbonate, rubidiumcarbonate, cesium carbonate, francium carbonate, beryllium carbonate,magnesium carbonate, calcium carbonate, strontium carbonate, bariumcarbonate, radium carbonate, or any mixture thereof.
 13. The method ofclaim 1, wherein the molten carbonate electrolyte comprises lithiumcarbonate.
 14. The method of claim 1, wherein the molten carbonateelectrolyte further comprises one or more additional oxygen, sulfur,halide, nitrogen or phosphorous containing inorganic salts.
 15. Themethod of claim 1, wherein the coulombic efficiency in step (c) isgreater than about 80%.
 16. The method of claim 1, wherein the methodresults in a coulombic efficiency in step (c) is about 100%.
 17. Themethod of claim 1, wherein the electrolysis reaction is performed at acurrent density of between about 5 and about 1000 mA cm².
 18. The methodof claim 1, wherein step (c) also produces molecular oxygen (O₂). 19.The method of claim 1, wherein the molten carbonate electrolytecomprises one or more transition metal nucleating agents, and during theelectrolysis of step (c), formation of transition metal nucleation siteson the cathode from the one or more transition metal nucleating agentsis suppressed by performing the electrolysis under conditions whichreduce the solubility of one or more of the transition metal nucleatingagents.
 20. The method of claim 19, wherein the transition metalnucleating agents for which solubility has been reduced are selectedfrom nickel, chromium, iron, and any combination of any of theforegoing.
 21. The method of claim 19, wherein the conditions forreducing the solubility of one or more transition metal nucleatingagents during electrolysis include (a) an electrolyte comprising (i) alithium carbonate and (ii) one or both of sodium carbonate and potassiumcarbonate, (b) decreasing the electrolysis temperature, (c) decreasingthe concentration of lithium in the electrolyte, (d) increasing theelectrolysis current density, or (e) any combination of any of theforegoing.
 22. The method of claim 1, wherein the method furthercomprises electrochemically exfoliating the carbon nano-onions from asecond anode to produce graphene having 1 to 5 layers.
 23. The method ofclaim 1, wherein the nano-onions are generated in the presence of zinc.24. A method for producing carbon nano-onions comprising: (a) heating acarbonate electrolyte comprising an oxide additive to obtain a freshlymelted carbonate electrolyte; (b) disposing the freshly melted carbonateelectrolyte between an electrolysis anode and an electrolysis cathode ina cell; (c) applying an electrical current to the electrolysis cathodeand the electrolysis anode in the cell to electrolyze the freshly meltedcarbonate and generate carbon nano-onions on the electrolysis cathode,without the formation of transition metal nucleation sites on thecathode.
 25. The method of claim 24, wherein the oxide additivecomprises lithium oxide.
 26. The method of claim 24, wherein the methodfurther comprises electrochemically exfoliating the carbon nano-onionsfrom a second anode to produce graphene having 1 to 5 layers.
 27. Themethod of claim 24, wherein the electrolysis cathode is coated withzinc.
 28. The method of claim 24, wherein the nano-onions are generatedin the presence of zinc.